1. Technical Field
The present disclosure generally relates to the audio signal amplification field and, more particularly, to an amplification circuit for driving an audio signal diffuser, such as, for example, a loudspeaker.
2. Description of the Related Art
Known audio amplifier systems generally include an audio signal source, an audio signal amplifier, a low-pass filter to remove frequencies higher than the audio band, and an audio signal diffuser (for example, a loudspeaker, headphones or earphones).
In particular, in such conventional systems, the signal of the voltage drop at the diffuser is biased to the zero value, enabling the diffuser to operate correctly and preventing damage to the diffuser.
This is achieved by using a decoupling capacitor connected between the terminal of the filtered audio signal and a terminal of the diffuser. Typical values of the capacitance of the capacitor are in the range from 100 μF to 220 μF, in order not to remove the audio frequencies (usually comprised between 20 Hz and 20 kHz).
The audio amplification system is supplied with a supply voltage V_alim and, afterwards, the audio amplification system is activated for diffusing an audio signal of an audio source (for example, a Compact Disk). Upon activation of the amplification system, the amplifier input starts being driven by an input audio signal (derived from the audio signal of the audio source), and the voltage signal at the amplifier output has an abrupt change from the zero value to a value greater than zero (for example an average value equal to half the supply voltage V_alim/2 of the audio amplification system 200). Initially, the decoupling capacitor is still discharged, and it starts charging slowly. The signal of the voltage drop at the diffuser undergoes an abrupt change from the zero value to a value greater than zero (in the example, an average value equal to half the supply voltage), which causes a noise generated by the diffuser, which is referred below to as “activation noise” (hereinafter referred to as “pop noise”).
The decoupling capacitor is then charged in a certain time interval, when the average value of the signal of the voltage drop at the diffuser decreases to zero. When in the steady state, the decoupling capacitor is charged to the working voltage value and the average value of the biasing voltage drop at the diffuser is zero, such that the diffuser can operate properly.
Methods for reducing the pop noise are known, whose object is to reduce the abrupt change of the signal of the voltage drop at the diffuser. These methods provide (where the pop noise is present) the use of a pre-charge step of the decoupling capacitor at the working value before the amplifier starts driving the diffuser with the amplified audio signal.
In this way, when the amplifier starts driving the diffuser, the decoupling capacitor is already charged at the working value and the biasing voltage drop at the diffuser is zero even when the amplifier starts driving the diffuser, thereby reducing the pop noise.
A prior method for implementing the decoupling capacitor pre-charging step is to use a resistive voltage divider located between the filtered amplified signal and the decoupling capacitor, by which an increasing voltage signal is generated, stepwise used to charge the decoupling capacitor from the zero value to the working value, thereby reducing the pop noise.
The applicant has realized that a drawback of this prior method is that it requires too much time to charge the decoupling capacitor (in the order of seconds), because the capacitance has a high value (in order not to remove the audio frequencies) and this time is proportional to the capacitance. Moreover, it is difficult to make the voltage divider by using two identical resistances and thus there is the risk that the decoupling capacitor is not being charged exactly at the working value, resulting in a pop noise that is only partially reduced.
A further prior method for the pre-charge of the decoupling capacitor is described in U.S. Patent Application No. 2008/0049952-A1, according to which it is possible to use a dedicated circuit to generate a voltage signal with a slowly increasing linear pattern to charge the decoupling capacitor. The dedicated circuit includes a width modulator that generates a pulse signal having a constant frequency and constant pulse width and includes an integrator to generate the linearly increasing signal as a function of the signal generated by the width modulator. Alternatively, the circuit includes a density modulator that generates a pulse signal with a constant pulse density and includes the integrator.
The drawbacks of this prior method is that it requires a dedicated circuit for the pre-charge of the decoupling capacitor and it requires an integrator to generate the pre-charging signal. Moreover, the applicant has recognized that this prior method further has the drawback that it requires too much time to charge the decoupling capacitor, because the signal used to charge the capacitor increases slowly over the time.